The present invention relates to a densitometer for measuring the density or concentration of suspended matter contained in fluid, such as the density or concentration of sludge, pulp, and various kinds of soluble matter contained in fluid, and in particular, a densitometer for measuring the density or concentration by using microwaves.
Conventionally, ultrasonic type densitometers and optical type densitometers have been widely used to measure the density or concentration of matter to be measured such as suspended matter contained in fluid. The ultrasonic type densitometer is designed to measure the attenuation of an ultrasonic wave, to thereby determine the density of the matter. The optical type densitometer is designed to measure the attenuation of a transmitted light or the rate at which a scattered light increases, to thereby determine the density of the matter.
The attenuation rate of an ultrasonic wave in air is far greater than in fluid. Accordingly, the attenuation rate of an ultrasonic wave is excessively increased when it is attenuated by bubbles mixing in fluid. Thus, it is considerably great, as compared with the case where an ultrasonic wave is attenuated by the suspended matter in the fluid. In such a manner, the measurement accuracy is greatly influenced by air. As a result, for example, measurement becomes impossible, or the measured density is higher than the actual density.
In order to prevent the influence of bubbles on the measurement, deforming type ultrasonic densitometers have been proposed. In this type of densitometer, fluid to be measured is taken into a pressurizing deforming chamber at a predetermined sampling cycle, and is then pressurized so that bubbles are dissolved in the fluid, and the fluid is measured. However, this method has the following disadvantages:
The mentioned density measurement cannot continuously be performed. Because the mentioned densitometer adapts the sampling method.
The matter to be measured needs to be sampled or pressurized, and thus a mechanical moving section for moving the matter is required, as a result of which the reliability worsens, and the maintenance is troublesome.
In the optical type densitometer, when dirt adheres to an optical window onto which light is emitted or is received, it has an effect on measurement, thus increasing the degree of error in measurement.
In recent years, a densitometer for measuring the density by using microwaves has been put to practical use as a densitometer which is hardly influenced by bubbles or dirt.
FIG. 1 shows the structure of a conventional densitometer using microwaves. Referring to FIG. 1, a microwave transmitting antenna 2 and a microwave receiving antenna 3 are provided on a detection pipe body 1 in which fluid flows, such that they are opposite to each other, and a microwave is emitted from a microwave oscillator 4. A first path is provided on which a microwave is transmitted through a power splitter 5, the transmitting antenna 2, the fluid in the pipe body 1, the receiving antenna 3, and a phase lag measuring circuit 6 in that order. In addition, a second path is provided on which a microwave is transmitted to the phase lag measuring circuit 6 only through the power splitter 5.
The above densitometer compares the phase lag .theta.2 of a microwave (represented by reference numeral 102 in FIG. 2) propagated through the detection pipe body 1 filled with the fluid to-be-measured via the first path with respect to a microwave (represented by reference numeral 100 in FIG. 2) transmitted via the second path, with the phase lag .theta.1 of a microwave (represented by reference numeral 101 in FIG. 2) transmitted through the detection pipe body 1 filled with a reference fluid such as city water with respect to the microwave (represented by reference numeral 100 in FIG. 2) transmitted via the second path. In this case, the microwave transmitted the detection pipe body 1 filled with the reference fluid is measured under the same condition as the microwave transmitted through the detection pipe body 1 filled with the fluid to be measured. Then, the phase lag .theta.1 is subtracted from the phase lag .theta.2 to determine the phase difference .DELTA..theta. (.DELTA..theta.=.theta.2-.theta.1).
The phase difference .DELTA..theta. is collated with a calibration curve indicating relationships between phase differences .DELTA..theta. and known density's, to thereby determine the density of the matter contained in fluid to be measured.
To be more specific, the relationship between the density and the phase difference is established to satisfy the following equation: EQU X=C.DELTA..theta. (1)
where X is the density, and C is a coefficient.
In such a manner, in order to determine the density, the densitometer using microwaves does not measure the attenuation of a microwave; it measures the phase difference (the difference between phase lags). Furthermore, in the densitometer, the density measurement is hardly influenced by bubbles or dirt. In other words, it can be correctly performed regardless of the bubbles or dirt, since the window portion on which a microwave is emitted or received does not need to be transparent, but may be dirtied. In addition, the density measurement can be continuously performed.
The phase lags .theta.1 and .theta.2 are set at optional values in the range of 0.degree. to 360.degree. in accordance with the density, etc. For example, suppose that the phase lag .theta.1 corresponding to a reference value (a density of 0) is 300.degree., and when the density varies by 5%, the phase difference .DELTA..theta. varies by 100.degree.. Under this supposition, the phase lag .theta.2 should be 400.degree., when fluid to be measured is made to flow into the pipe body 1, and a microwave is transmitted to the fluid.
However, the phase lag .theta.2 is apparently 40.degree. since the densitometer indicates the phase lag in the range of 0.degree. to 360.degree..
More specifically, in the above densitometer (using microwaves), even when the phase lag rapidly varies from a value (e.g., 260.degree. to 360.degree.) close to 360.degree. to a value (e.g., 0 to 100.degree.) close to 0.degree., it varies actually successively from the value close to 360.degree. to 359.degree., from 359.degree. to 360.degree. (0.degree.), from 360.degree. (0.degree.) to 1.degree., and from 1.degree. to the value close to 0.degree.. In this case, when the phase lag varies from 360.degree. (0.degree.) to 1.degree., it is regarded that it enters the "first rotation".
In this case, as a matter of convenience, the "rotation" is defined as follows: when the phase lag .theta.2 is 0.degree. or more and 360.degree. or less (0.degree..ltoreq..theta.2.ltoreq.360.degree.), it is determined as a value of the "zero rotation"; when the phase lag .theta.2 is more than 360.degree. and 720.degree. or more (360.degree.&lt;.theta.2.ltoreq.360.degree.), it is determined as a value of the "first rotation"; and when the phase lag .theta.2 is more than 720.degree. and 1080.degree. or more (720.degree.&lt;.theta.2.ltoreq.1080.degree.), it is determined as a value of the "second rotation". In other words, when the phase lag .theta.2 is (N-1).times.360.degree. or more and n.times.360.degree. or less ((N-1).times.360.degree..ltoreq..theta.2.ltoreq.n.times.360.degree.), it is determined as a value of the "(n-1)-th rotation" (n=an integer), and the phase lag .theta.1 is determined as a value of the "zero rotation".
In the above case, it is determined that as shown in FIG. 3, the phase lag .theta.2 has shifted from the first range of 0.degree. to 360.degree. (which is indicated by "N=0" in FIG. 3) to the second range of 0.degree. to 360.degree. (which is indicated by "N=1" in FIG. 3). Therefore, a correcting arithmetic operation is performed to correct the phase lag .theta.2. To be more specific, it is performed such that the phase difference .DELTA..theta. satisfies the following equation (equation 2): EQU .DELTA..theta.=.theta.2'+360.times.N-.theta.1 (2)
where .theta.2' is an apparent phase value, and N is the number of rotations (N=an integer).
In the above case, it is determined that the number N has increased by 1. This concept, as disclosed in Japanese application No. 5-171576, is given under a general process control condition wherein the density of matter to be measured in fluid varies continuously, and does not rapidly vary for a short measurement time period (e.g., five seconds).
In the above densitometer using microwaves, when the fluid in the pipe body 1 is discharged therefrom until the body 1 empties, the density measurement cannot be performed, and needless to say, the density does not continuously vary. In other words, the aforementioned general process control condition is not satisfied. As a result, there is a possibility that the number N of rotations may not be correctly counted. In such a case, the counted number N indicates an excessively high or low value. For example, when the number N=0, if it is mistakenly determined that N=2, the phase difference .DELTA..theta. is also mistakenly determined in the following manner: the phase lag .theta.1 is subtracted from the phase lag .theta.2' to obtain a value, and 720.degree. is added to the value to determine the phase difference .DELTA..theta.. Actually, the phase difference .DELTA..theta. should be determined simply by subtracting the phase lag .theta.1 from the phase lag .theta.2'. In other words, 720.degree. should not be added. Accordingly, the above mistakenly determined density is higher than the actual density by 720.degree. (360.degree..times.2).
Furthermore, if the number N of rotations is still incorrect (for example, it is still determined as 2), the density measurement cannot be correctly performed even if the pipe body is re-filled with fluid after discharge of fluid from the pipe body.
The object of the present invention is to provide a densitometer using microwaves, wherein the density measurement can be correctly performed even if fluid is discharged from a pipe body or fluid is re-filled into the body after discharge of the previously filled fluid therefrom.